Council diplomats have said privately that the US and Gulf Arab states have become increasingly frustrated in recent weeks with what they saw as Annan's determined commitment to diplomacy at a time when they believe all avenues for dialogue with Assad have been exhausted.

Ban said in the statement that he and El Araby were in discussions on appointing a successor to Annan.

"Kofi Annan deserves our profound admiration for the selfless way in which he has put his formidable skills andprestige to this most difficult and potentially thankless of assignments," Ban said.

'Finger-pointing'

Al Jazeera's Kristen Saloomey, reporting from UN headquarters in New York, said it is unclear "what impact this will have on Friday's vote" when the UN General Assembly rule on a Saudi-drafted resolution on Syria.

Vitaly Churkin, Russia's UN ambassador, was quick to react, suggesting to reporters outside the Security Council in New York that Moscow was disappointed that Annan was bowing out.

"We understand that it's his decision," Churkin told reporters. "We regret that he chose to do so. We have supported very strongly Kofi Annan's efforts.

He has another month to go, and I hope this month is going to be used as effectively as possible under these very difficult circumstances."

Churkin added he was encouraged that Ban was looking for a successor to Annan.

Churkin suggested that Western powers that opposed "reasonable and balanced proposals" in the Security Council undermined Annan's peace efforts from the start.

Aleppo 'under siege'

Western diplomats, however, blame Russia and China for vetoing three council resolutions intended to increase the pressure on Assad to stop his attacks on civilians and halt the fighting, which has escalated into a full-scale civil war.

"His [Annan's] resignation highlights the failure in the United Nations Security Council of Russia and China to support meaningful resolutions against Assad that would have held Assad accountable for his failure to abide by the Annan plan," White House spokesman Jay Carney said.

"Those vetoes... were highly regrettable and placed both Russia and China on the wrong side of history and the wrong side of the Syrian people."

Doctors in Turkey say they need more help to treat the injured fighters arriving from Syria [Al Jazeera]

The grant brings to $76m the total amount of food, water, medical supplies, clothing, hygiene kits and other aid to some of the 1.5 million people in need in Syria, the White House said in a statement.

According to the Reuters news agency, Obama has also signed a secret order authorising US support for rebels seeking to depose Assad and his government.

Obama's order, approved earlier this year and known as an intelligence "finding", broadly permits the Central Intelligence Agency and other US agencies to provide aid that could help the rebels dislodge Assad from power.

In another development on Thursday, Syrian rebels shelled an airbase used by regime forces to pound the northern city of Aleppo, with a human rights watchdog reporting the killing of dozens near Damascus in a series of government raids.

"Menagh military airport was bombarded on Thursday morning by a tank captured previously by the rebels," the Syrian Observatory for Human Rights said of the base 30km northwest of Aleppo, the country's commercial capital.

"Aleppo is under siege and the prospect of the loss of thousands more civilian lives in Syria is very high," Annan wrote in the Financial Times article.